UV curable multilayer paint coatings are applied to a substrate through spraying, screen printing, dipping or brushing for the protection and decoration of the substrate. In the usual application, a substrate, such as metal, glass or plastic is successively coated to form at least three distinct layers, commonly referred to as the primer, base and top coat layers. As each layer is applied to the substrate, ultraviolet light is introduced to cure the layer. Multilayer paint coatings using UV curing processes offer many advantages over typical heat curable compositions.
Heat curable compositions require the use of organic solvents that contain a significant amount of volatile organic compounds (VOCs). These VOCs escape into the atmosphere while the heat curable composition dries. Such solvent based systems are undesirable because of the hazards and expense associated with VOCs. The hazards include water and air pollution and the expenses include the cost of complying with strict government regulation on solvent emission levels. In contrast, UV curable compositions contain reactive monomers instead of solvents; thus eliminating the detrimental effects of the VOCs.
Additionally, the process of heat curing typically results in coatings which suffer from poor physical properties. In the typical heat curing process, the base coat must be inordinately thick in order to hide the primer coat. Additionally, the thick base coat must be dried for a significant time to eliminate intermixing between the base coat and top coat. Due to the thickness and significant drying time of the base coat, the resulting multilayer coating suffers from low resistence to chipping and a lack of surface smoothness.
Although UV curable compositions exhibit superior properties and performance over their heat curable counterparts, UV curable compositions themselves suffer from certain disadvantages. Generally, UV curable compositions have high molecular weights and a substantial degree of cross linkage due to the highly reactive nature of the composition. As a result, many of these compositions suffer from low durability and resin shrinkage. With the use of many such compositions, an inordinately high amount of UV light is required to cure. New formulations that lessen these problems typically suffer from diminished abrasion, chemical, and scratch resistance as well as low thermal stability and adhesion.
Accordingly, there exists a need to provide environmentally safe UV curable multilayer coating paint compositions which exhibit improved appearance, weatherability, corrosion resistance and workability. Additionally, there is a need to provide a method of applying an improved composition which furthers the goal of improved performance.